early 15c., from Lowland Scottish, from Gaelic bealltainn "May 1," important Celtic religious rite marking the start of summer, probably literally "blazing fire," from PIE root *bhel- (1) "to shine, flash, burn" (see bleach (v.)) + Old Irish ten "fire," from PIE *tepnos, related to Latin tepidus "warm." But this derivation of the second element is hotly disputed by some on philological grounds, and fires were equally important in the other Celtic holidays.

The rubbish about Baal, Bel, Belus imported into the word from the Old Testament and classical antiquity, is outside the scope of scientific etymology. [OED]

Also known as "Old May Day," because after the 1752 calendar reform it continued to be reckoned according to Old Style; it was one of the quarter-days of ancient Scotland.